The present invention relates to dynamoelectric machines and, more particularly to rotors of AC dynamoelectric machines.
The dynamoelectric machine may be either a motor or a generator. For concreteness, the present invention is described in the environment of a motor. It will be understood that the following description is equally applicable to a generator.
A conventional motor employs a rotor made of a solid forging of magnetic metal in which field windings are inset into longitudinal slots for carrying an energizing DC current to produce magnetic poles in the rotor. The rotor spins in the center of a stator, typically formed by stacking a large number of laminations of magnetic metal mutually insulated from one another. The stator includes windings energized by AC currents to produce a magnetic field which rotates about the axis of the rotor at a predetermined angular rate.
It is recognized that a time-varying magnetic field is capable of inducing eddy currents in an electrically conductive metal. Most useful magnetic metals exhibit relatively high resistivity. As a result, eddy currents induced in such metals are capable of dissipating large amounts of energy in resistive heating. The strategy for avoiding eddy current losses is different for the stator and rotor of a motor.
At any point on the stator of a motor, a time-varying magnetic field must exist in order to produce the required rotating magnetic field upon which the operation of the machine is based. Excessive eddy current losses are avoided to a large extent in the magnetic mass of the stator by building up the stator from relatively thin, mutually insulated laminations of magnetic metal. The thinness of each lamination, and its insulation from its neighbors, provides an extremely resistive path for severely limiting eddy currents induced therein.
The technique of using laminations to reduce the effects of eddy currents is not available in rotors of large dynamoelectric machines, since the stresses imposed by rapid rotation of a large-diameter rotor cannot be withstood by a laminated structure. Accordingly, it is conventional to form a rotor from a one-piece forging of high-strength magnetic metal. Longitudinal slots are machined in the forging to receive conductors for establishing the magnetic field. At the ends of the rotor, end turns interconnect the conductors. The conductors are held in the slots against the substantial centrifugal acceleration by dovetail wedges driven into dovetail slots near the surfaces of the slots. The end turns are held against centrifugal acceleration by high-strength retaining rings shrink fitted onto the ends of the forging.
Accordingly, the rotor is essentially a large mass of magnetic material in which substantial eddy currents could be generated in the presence of time varying magnetic fields. The retaining rings likewise could be subject to eddy currents.
Eddy currents are not usually a problem in a rotor since the rotor spins synchronously with the rotating magnetic field produced by the stator. Thus, except for relatively small variations due to load variations, each point on the rotor experiences a substantially constant magnetic field. As a consequence, eddy currents are not generated.
Some types of motors employ synthesized waveforms to control an AC motor from a DC source or from an AC source whose frequency is different from that which the motor requires. In one common situation, it is desired to drive an AC induction motor at variable speeds from a constant-frequency AC source. One technique for varying a motor speed includes reducing the amplitude of a constant frequency power source, thereby permitting the motor to slip behind synchronism with the power-source frequency. Unfortunately, as the rotor slips far out of synchronism with the rotating magnetic field, the motor becomes unable to generate a substantial torque. Beyond about six percent difference in the speeds of the rotating magnetic field and the rotor, the torque drops close to zero. Thus, permitting the rotor to slip behind the frequency of the rotating magnetic field has limited utility in many applications.
In a load commutated inverter, a DC source is inverted using switching devices such as, for example, thyristors or silicon-controlled rectifiers, to synthesize an AC power supply having the desired frequency and an approximation of the desired amplitude for application to the stator. Since switching devices are employed to synthesize the AC supply, the AC power thus derived, in addition to a desired fundamental frequency, also contains substantial undesired harmonics. Thus, although a rotor driven at a speed determined by the fundamental frequency of such a supply sees a substantially constant magnetic field from the fundamental frequency, it is nonetheless exposed to substantial variations in magnetic field at the harmonic frequencies. Such harmonic frequencies range from about a few hundred Hz to one or more tens of KHz.
If the available primary power source is an AC source, a load commutated inverter employs a rectifier and filter to produce the required DC prior to inversion.
A cycloconverter produces an AC power output directly from an AC primary source by controlling and switching the AC power to synthesize the desired output waveform. The output waveform, although it contains a fundamental frequency, is also accompanied by substantial higher-frequency harmonics. As in the case of the load commutated inverter, these harmonics can give rise to eddy currents in the rotor, with resultant power losses and heating.